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Rami Arieli: "The Laser Adventure"
The excitation mechanism
The excitation mechanism is the source of energy that raises the atoms in the active
medium into their excited state, thus creating population inversion.
In accordance to the law of conservation of energy, the electromagnetic radiation out of
the laser is always less then the input energy by the excitation mechanism. There are
lasers with efficiency of much less than 1% (!), while others with efficiencies
approaching a 100%.
There are few types of excitation mechanisms:
a) Optical pumping - Excitation by photons:
In lasers with solid or liquid active medium, it is common to supply the excitation
energy in a form of electromagnetic radiation (photons) which are absorbed in the
active medium.
The electromagnetic radiation source can be of different kinds:
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Flash lamps, which are build from a quartz tube filled with gas at low pressure.
Usually Xenon gas is used, but sometimes when higher energy is required, other
noble gasses with lower atomic weights such as Krypton or Helium are used.
Another laser or any other light source such as the light from the sun.
b) Electrical excitation of a gas:
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When the active medium is in the gas state, the best excitation is by electrical
discharge of the gas (see figure 3.3).
Figure 3.3: Electrical Excitation of a Gas Laser
The gas in the tube is electrically neutral, and as long as no external energy is
applied, most of the molecules are in the ground state.
When the high electrical voltage is applied, electrons are released from the
cathode and accelerated toward the anode. On their way, these electrons collide
with the gas molecules and transfer energy to them. Thus, the gas molecules
are raised to excited state.
Higher voltage is required to start the electrical discharge in the tube than to
keep the discharge. Thus, a preliminary high voltage pulse is applied for initial
discharge, and then the voltage is lowered to its operating value.
The ballast resistor is used to limit the current in the tube after discharge is
achieved.
c) Collisions with atoms
This is the standard excitation mechanism in the commercial gas lasers such as:
Helium-Neon laser, or Carbon-Dioxide laser.
In this method at least two gasses are inside the laser tube.
One gas receives the energy from the collision with the accelerated free electrons.
The second gas receives energy from collisions with the excited molecules of the
first gas.
d) Chemical Excitation
In this excitation, the energy is supplied by the chemical reaction between two
atoms or molecules
e) Electric current in diode lasers